Tone not sure exactly what you mean by "Aerofoil", but there is a reason why all airplanes and boat propellers have the same shape.

Friction.

There is a huge amount of friction if you build a foil and you make it a square box.

Now there's no particular reason you have to make it exactly the same. Just like how there are some airplanes with two wings like the biplane or airplanes with the propeller in the back (pusher airplanes) swept wing aircraft, gliders with long wings but they all have in common an aerodynamic shape which lets air pass it without much friction.

You can't just cut a square plank and stick it in the water. make sure you round the edges, smooth out the joins, do stuff like that and you can find plenty of room to experiment.

does it have to look like a little airplane? No. Does it have to share the properties of an airplane such that it glides easily through the water? yes.

Arcsrule wrote:the foil must generate lift. a doubt 'flat' will generate enough lift to make you smile. i'm not an engineer but i have slept in a Holiday Inn Express.

This is not true. Check out any angle of attack / lift coefficient diagram for instance here's one with a cesna and british jet

Cl is about .3 at 0 for a cesna and actually there is no lift in a jet flying flat without any angle of attack. Yet it flies!

Angle of attack is the #1 reason that planes have lift and are able to fly. More angle of attack = higher lift coefficient = you climb up in the air.

Now the question remains why are airplane wings shaped like they are and the reason is drag. If you can get the plane to have a bit more lift with a lower angle of attack this means you will have less drag.

The purpose of the shape of an airplane wing is to reduce drag. Sure the camber creates a little lift but the main purpose is not to lift it into the air (angle of attack does that), it is mean to reduce drag and make the plane fly faster and more efficiently by allowing the plane to have a lower angle of attack than it would otherwise have for that Cl.

Hydrofoils are the same way. Water is a lot thicker than air, so it is very important to create shapes that don't have much drag.

Does it need camber? No of course not. Design your hydrofoil and don't be afraid to experiment. You don't need camber but pay attention to how much drag your shape is making.

Tiago1973 wrote:i wonder if in practical terms there is tons of a difference between a crude airfoil shape and a flat plate

Not "tons". But enough that the only airplanes that have symmetric airfoils are supersonic airplanes and stunt planes. Then there are boat props, airplane props, helicopter propellers, some are symmetric some not, depending.